Plastics fabrication techniques, such as extrusion blow molding, injection blow molding, profile extrusion, pipe extrusion, blown film extrusion, co-extrusion (with a second plastic material), extrusion coating, foam molding, foam extrusion, thermoforming, and the like, all require the plastic composition to have a high melt viscosity and melt strength (melt elasticity) during processing.
Linear polyesters have a low melt elasticity and are therefore not suitable for these applications.
It has now been discovered that introduction of even a low degree of branching causes the necessary increase in melt elasticity and melt viscosity at low shear rates, which make these modified polyesters uniquely suitable for use in the applications mentioned.
Blow molding is used to make hollow shaped plastic articles in a variety of commonly encountered forms, such as milk bottles, auto windshield washer tanks, street light globes, arms and legs on toy dolls, and a multitude of others. There are two basic types of blow molding processes, both being fundamentally related, but technologically dissimilar. Extrusion blow molding typically comprises extruding a tube of plastic into a water-cooled mold, inflating the tube by internally introducing air or another gas until the walls of the molten tube assume the shape of the mold, allowing the shaped tube to cool to structural rigidity, and removing the extrusion blow molded part from the mold.
Another major technique, injection blow molding, involves, instead of extrusion, injection molding the plastic around core pins in an injection mold, then transferring to the blow mold. The fundamental difference between injection blow molding and extrusion blow molding is that with the former, two complete sets of molds are required--as injection mold for molding the preform and a blow mold for the final form.
Until now, polyolefin resins have been the plastics of choice for extrusion blow molding and while injection blow molding can use a greater variety of resins, even including poly(vinyl chloride) resins, not all of them can be substituted into the extrusion blow molding process because of a fundamental lack of melt strength and elasticity.
Most thermoplastics, and even polyolefins, have their shortcomings in extrusion blow molding, especially if the preformed molten tubes (parisons) are too big and heavy. The tubes stretch and become difficult to handle and special equipment is needed to prevent this hot stretch. Moreover, the parts must often be removed from the blow mold while still warm and supported on special "cool-down" fixtures to avoid tearing, etc.
Injection blow molding is illustrated in the drawing. At position "A:", the hot plastic is injected into a hot mold around a pin, the preform being called a "parison." The outer mold is removed, and the "parison", still molten, is rotated to position "B", where it is surrounded by a mold in the shape of the final part. The molten plastic is now inflated (e.g., by air) to fill this mold, and is cooled to solidify. Inflation of the polymer melt is the critical step, requiring good melt elasticity for its successful completion. Linear polyesters, e.g., polybutylene terephthalate, in such an application, tend to run, droop, and/or "burst", making it impossible to obtain a useful part.
After an additional 120.degree. rotation, position "C", the final part, is removed from the mold (step C). Obviously, this technique is distinct from ordinary injection molding, in which a molten plastic is injected into a mold and cooled without further processing. There is no concern at all there with the need for self-support in the molten preform.
Other "blow" techniques have in common with the injection blow molding process that the molten polymer is inflated (by air, or a suitable inert gas) to assume its final desired shape. In extrusion blow molding an extruded tube is inflated inside a mold; in blown film extrusion an extruded tube is continuously inflated to a large diameter tube of low well thickness, which is subsequently collapsed and further processed to yield film, (grocery) bags, etc., and in foam molding or foam extrusion applications a cellular structure is introduced in the plastic through expansion of an inert gas, again requiring high melt elasticity to prevent collapse of the foam before the part has solidified.
In extrusion of profiles of closely controlled shape and dimensions it is important that the molten plastic upon leaving the extruder die does not sag or drool until it has hardened or solidified. Polyesters of the branched structure have been now discovered to have a high melt viscosity under the low shear forces acting on the extrudate, preventing sagging and drooling; at the same time, they exhibit significantly lower melt viscosities under the high shear rate conditions existing in the extruder die, facilitating the passage of the melt through the die without requiring excessive pressures in the extruder barrel. The branched polyesters are therefore uniquely suited for precision extrusion applications such as are required for profile and tubing extrusions, extrusion coatings and coextrusions.
In thermoforming, a sheet of plastic is suspended horizontally over a suitable mold and heated, usually by radiant heat, until melted. The sheet is then brought into contact with the mold and collapsed onto it by suction. After cooling, the plastic, which has assumed the shape of the mold is lifted off, trimmed and decorated as desired.
Obviously, this application too calls for a high degree of elasticity of the polymer melt to prevent premature sagging and running of the material.
Shaped articles having excellent appearance and physical properties can be obtained if branched, high molecular weight poly(alkylene terephthalate) resins are used as the thermoplastic. In admixture with another thermoplastic, sufficient branched such polyesters will be used to provide melt strength and elasticity. More particularly, if a branching component with functionality of greater than 2, i.e., at least 3, is used in a reaction with either a dialkyl terephthalate or terephthalic acid, and an alkylene glycol as a component of the reaction mixture there is formed a highly branched polyester with properties allowing its use in techniques where a self-supporting preform is essential. The relative degree of branching is indicated by the relative diameter during extrusion, due to die swell of the material relative to unbranched materials. The branched materials show a surprising and unexpected improvement in melt strength in comparison with the unbranched materials.
Although similar polyesters are the subject matter of U.S. Pat. No. 3,692,744, these are described and claimed in the form of injection molded articles. They are characterized in the patent as providing molded articles having improved shock resistance. There is no hint or suggestion in the patent that the branched polyesters will have improved processability during molding and there is no suggestion that the branched polyester will have improved melt characteristics which makes them suitable for techniques where self-supporting preforms with high melt strengths are essential. Thus, even though applicants contemplate using quite similar plastics compositions, it is the discovery of their unique utility in the fields specified above which is the primary basis for patentability over the injection molded, high impact strength compositions described in U.S. Pat. No. 3,692,744.